The Human Eye and the Colourful World — RBSE Class 10 (Science)
Your eye is a self-focusing camera that adjusts in an instant from a book to a distant hill. And the same light it captures paints the sky blue by day and the sun red at dusk. This chapter applies the optics you have just learned to the eye that sees — and to the atmosphere that colours everything.
1. The human eye and how it focuses
Light enters through the cornea, passes the pupil (whose size the iris controls), and the eye lens focuses it onto the light-sensitive retina; nerve signals travel to the brain via the optic nerve.
Accommodation is the eye lens's ability to change its focal length (by the ciliary muscles changing its curvature) to focus near and far objects. For a normal eye:
- Far point = infinity; near point (least distance of distinct vision) = 25 cm.
2. Defects of vision and their correction
| Defect | Problem | Cause | Corrected by |
|---|---|---|---|
| Myopia (short-sight) | can't see distant objects; image forms before retina | eyeball too long / lens too converging | concave lens |
| Hypermetropia (long-sight) | can't see near objects; image forms behind retina | eyeball too short / lens too weak | convex lens |
| Presbyopia | near vision weakens with age | ciliary muscles weaken, lens stiffens | convex (often bifocal) |
A cataract (clouding of the lens) is treated by surgery, not lenses.
3. Refraction through a prism and dispersion
A prism bends light twice; the total bend is the angle of deviation. Crucially, different colours bend by different amounts, so white light splits into its colours — dispersion — giving the spectrum VIBGYOR (violet bends most, red least). A rainbow is nature's dispersion: raindrops refract, disperse and internally reflect sunlight.
Recombining the spectrum with a second, inverted prism gives back white light (Newton's experiment).
4. Atmospheric refraction
The atmosphere's varying density bends light, causing:
- Twinkling of stars — starlight refracts through fluctuating air layers, so the apparent brightness/position wobbles. (Planets, being nearer and disc-like, twinkle far less.)
- Advanced sunrise and delayed sunset — the sun is visible about 2 minutes before it actually rises and after it sets, because light bends over the horizon.
5. Scattering of light — why the sky is blue
Tiny particles/molecules scatter light; shorter wavelengths (blue) scatter much more than longer ones (red).
- Blue sky — sunlight's blue is scattered across the sky by air molecules.
- Red sunrise/sunset — near the horizon light travels a long path; most blue is scattered away, so the sun and sky look reddish.
- The Tyndall effect — scattering by larger particles (smoke, fog, colloids) makes a light beam's path visible.
- The sky would look black with no atmosphere (as astronauts see) — no molecules to scatter light.
6. Closing thought
The eye is optics made personal: accommodation focuses, and each vision defect is fixed by the right lens (concave for myopia, convex for hypermetropia). The "colourful world" is dispersion and scattering — blue skies, red sunsets, twinkling stars. Learn the defect-cause-correction table and the scattering reasons cold. In the RBSE board this chapter reliably yields a diagram/defect question and 1-mark reasoning questions worth 5–6 marks.
